Optical and structural study of GaN nanowires grown by catalyst-free molecular beam epitaxy. I. Near-band-edge luminescence and strain effects

GaN nanowires with diameters of 50-250 nm, grown by catalyst-free molecular beam epitaxy, were characterized by photoluminescence (PL) and cathodoluminescence (CL) spectroscopy at temperatures from 3 to 297 K, and high-resolution x-ray diffraction (HRXRD) at approximate to 297 K. The lattice parameters of the nanowires, determined by HRXRD, are in good agreement with recent measurements of freestanding quasisubstrates; the relative variation of the lattice parameters between the nanowires and quasisubstrates is <= 2x10(-4). Both as-grown samples, which contained nanowires oriented normal to the substrate as well as a rough, faceted matrix layer, and dispersions of the nanowires onto other substrates, were examined by PL and CL. The (DXA)-X-0 line at 3.472 eV, ascribed to excitons bound to shallow donors, was observed in low-temperature PL and CL; free-exciton lines (X-A at approximate to 3.479 eV, X-B at approximate to 3.484 eV) were observed in PL at temperatures between 20 and 80 K. The linewidth of the (DXA)-X-0 peak was larger in PL spectra of the nanowires than in quasisubstrates. The broadening of the (DXA)-X-0 peak in PL of the nanowires is tentatively ascribed to inhomogeneous stress/strain. In addition, the (DXA)-X-0 peak was significantly broader in CL than in PL spectra of the same nanowire samples. The further large broadening of the CL peak (as compared to PL) is tentatively ascribed to Stark effect broadening, induced by the electric fields of trapped charges that are created in the CL excitation process. (c) 2007 American Institute of Physics.